This invention relates generally to ram-type blowout preventers, and, more particularly, to actuators for the rams of a blowout preventer either for shearing a pipe extending through the bore of the blowout preventer and then sealing across the bore or for sealing around a pipe extending through the bore. Still more particularly, the invention relates to an apparatus for enhancing the pressure area and limiting the stroke of the operating piston of the actuator.
As is well known in the art, a ram-type blowout preventer composes a housing connected to a wellhead with the housing having a bore which is in alignment with the wellbore. Rams are movable within guideways extending transversely from the bore between an outer position removed from the bore and inner position across the bore and engaging with one another to seal off the bore. In a pipe ram blowout preventer, the inner ends of the rams have recesses for sealing around the pipe suspended within the bore. In a shear ram blow-out preventer, the inner ends of the rams include blades to shear pipe and also seal which may be flat or otherwise complimentary for sealing across the open bore after the pipe is sheared. Upon inward movement of the rams into the bore, the sealing engagement between the seals carried on the rams effectively terminates any fluid flow through bore.
A hydraulically actuated cylinder having a piston interconnected to the respective ram by means of a shaft or a "stem" effects the movement of the rams into sealing engagement. The stem is provide with stem packings or seals disposed about the stem to prevent pressure and fluid in the bore from being communicated along the stem into the hydraulic circuit of cylinder.
The actuator is rigidly affixed to one end of the housing and extends outwardly in the same direction on either side of a respective ram stem. The actuator includes a bonnet in which is disposed a piston and cylinder with the bonnet being affixed to the housing by means of bonnet bolts. The cylinder is connected to a hydraulic circuit for providing fluid under pressure to move the piston within the cylinder. The piston is disposed on the stem to reciprocate the ram.
Typical shear ram blowout preventers are shown in U.S. Pat. Nos. 3,946,806; 4,043,389; 4,313,496; and 4,132,267. Typical pipe ram blowout preventers are shown in U.S. Pat. Nos. 4,492,359 and 4,504,037. A preferred ram-type blowout preventer is shown on pages 637-645 of the Composite Catalog of Oilfield Equipment and Services, 1992-93, published by World Oil.
As can be appreciated, a substantial pressure force must be applied to the shear rams of a shear ram blowout preventer to shear the pipe extending through the bore of the preventer. Various means have been employed in the prior art actuator to ensure that the actuator provided adequate shear force. For example, a booster piston and cylinder may be piggy backed onto the primary operating piston and cylinder to ensure sufficient force to both shear the pipe and seal the bore.
The actuators for the rams are designed for removal of the rams in the field to change the pipe rams or shear rams of the blowout preventer. One method is the use of ram change pistons and cylinders whereupon removal of the bonnet bolts, the ram change pistons and cylinders are hydraulically actuated causing the bonnet to travel radially inward and outward with respect to the blowout preventer housing in a manner like that of the ram operating piston. Upon moving the bonnet into the outward position because it is no longer restrained by the bonnet bolts, the pipe ram or shear ram, as the case may be, is also pulled along the ram guideway outwards of the blowout preventer bore so as to be removed from the housing and thus exposed and rendered accessible for servicing or replacement.
It is often desirable to either replace a pipe ram with a shear ram or vice versa. Since the stroke of a shear ram is longer than the stroke of a pipe ram, it is necessary that a plurality of parts be replaced to modify the actuator to accommodate another type of ram. For example, a much greater force is required on a shear ram than on a pipe ram since the shear ram must not only shear the pipe extending through the blowout preventer housing bore but also must then seal the resulting open bore. Thus, in adapting an actuator previously used for a pipe ram, the standard bonnet must be replaced with a large bore shear bonnet to provide a larger capacity operating piston to increase force on the shear ram to shear the pipe. The adaptation of the actuator for a shear ram, particularly when done in the field, is complicated and time consuming.
Further, the substantial operating piston force required to shear the pipe in a shear ramtype blowout preventer is much greater than the amount of force required to seal the open bore after the pipe has been sheared. This over capacity force on the elastomeric seals of the shear ram reduces the life of the seals. It is preferred that the operating piston force on the seals be reduced after the shear rams have sheared the pipe. However, the hydraulic pressure on the operating piston frequently remains at the same high level during both the shearing and sealing operations.
The present invention overcomes the deficiencies of the prior art.